In the present study, data from more than 550 periodontally diseased patients with more than 1100 furcation invasions were retrospectively analysed. There were apparent differences in the distribution of different furcation degrees in patient populations treated by 2 differently experienced operators. However, treatment modality patterns were rather similar. Scaling during periodontal flap surgery was the most often performed treatment procedure in degree I (97-98%) and II (75-83%) involvements. About 44% of degree III involved teeth were extracted. In order to determine the influence of degree of furcation involvement, tooth type and operator variability on treatment modality, logistic regression analysis was applied. Degree of furcation involvement was an important indicator variable in all models. Scaling as a sole measure was mainly performed in relation to degree I of furcation involvement. With every increase in degree, the odds of scaling decreased by factor 12.7. The odds of root resection was upper 1st molars 46 x higher than in wisdom teeth or lower 2nd molars with the same degree of involvement, but only 3.3 x higher than in lower 1st molars. Tunnel preparation as well as regenerative procedures were mainly confirmed to lower molars. Operator variability was only introduced as a covariate in the extraction model. hence, despite of different operator skill and severity of periodontal disease in treated populations, decision for one or the other treatment modality seems to depend essentially on degree of furcation involvement as well as tooth type.